This invention relates to video processing and more particularly to techniques for converting video signals from one field rate to another and to apparatus making use of such techniques.
One important application lies in the conversion of signals to and from high definition television (HDTV) formats. The SMPTE 240M HDTV system has a field rate of exactly 60 Hz. As will be later described in more detail, there are a number of important applications in which it is necessary to convert an HDTV signal to current broadcast standards. In the case of NTSC, this will result in a conversion in field rate from 60.00 Hz to 59.94 Hz and it has already been recognised that this conversion requires a field to be dropped every 16 seconds. Conversely, instead of down-converting a signal from 60.00 Hz HDTV to 59.94 Hz NTSC, there are applications which require up-conversion from 59.94 Hz NTSC to 60.00 Hz HDTV. It will then be necessary to repeat a field every 16 seconds.
In certain applications, whether involving conversion of field rates between 60.00 Hz and 59.94 Hz or other closely spaced field rates, this intermittent dropping or repeating of a field is perceptible and visually objectionable. There exists the alternative, using current techniques, of interpolating in the manner of a broadcast standards converter from one field rate to the other. Inevitably, however such interpolation produces a softening of the visual image. This too can be objectionable.
It is an object of the present invention to provide a method for converting video signals between field rates, usually closely spaced, that avoids abrupt and perceptible field transitions without introducing appreciable softening of the overall image.
Accordingly, the present invention consists in one aspect in a method of converting a video signal between field rates of respectively F and F+/xe2x88x92xcex94 per second, wherein conversion is performed in a synchronization mode over a first portion of the time interval 1/xcex94 and performed in an interpolation mode over a second portion of said time interval.
By xe2x80x9csynchronizationxe2x80x9d is meant the creation of output fields without temporal interpolation, or at least without temporal interpolation of low spatial frequencies. It will be recognized that a converter may take contributions from adjacent fields in order to improve the high frequency performance, butxe2x80x94provided there is no temporal interpolation of low frequency informationxe2x80x94blurring or smoothing of the picture will be avoided.
Preferably, the phase difference between input and output fields is monitored and the interpolation mode is invoked as the phase difference exceeds a predetermined threshold.
Advantageously, the interpolation mode is sustained for a fixed time interval after it is invoked.
The present invention will find particular application in HDTV up- and down-converters, where elements of the hardware necessary to conduct interpolation between fields, may serve also in improving the line interpolation conducted between 1125 and 525 lines. There will be other applications, however, in converting video signals between closely spaced field rates, where it is desirable to avoid abrupt and perceptible field transitions and where the additional costs involved in conducting interpolation are justified in terms of the performance demanded. One such application will be high specification synchronizers.
The SMPTE 240M HDTV system has a similar picture quality to 35 mm cinematographic film. Even before widespread broadcasting of HDTV, the availability of HDTV receivers and domestic HDTV laser disc players will create a market for film material in the HDTV format. There accordingly exists the massive task of transferring 16 mm and 35 mm film material to HDTV.
The conversion of video material between NTSC and PAL broadcast standards is commonplace. The NTSC standard has a field rate of 59.94 Hz compared with the 50 Hz field rate of the PAL standard. A broadcast quality standards converter will utilize a field aperture of four fields or more, to conduct field interpolation between the 59.94 Hz and 50 Hz field rates.
It is a frequent requirement to produce NTSC or PAL video material in a telecine operation from 16 mm or 35 mm cinematographic film. With rare exceptions, film is intended to run at 24 frames per second. To convert from the resulting field rate of 48 Hz to the NTSC standard, the well-known xe2x80x9c3-2 pull-down systemxe2x80x9d is used to duplicate certain fields. The resulting NTSC signal, while having a slightly stuttery motion, is acceptable for broadcast. A PAL video signal can be created in a telecine operation from cinematographic film with an appropriate modification to running speed to convert from 48 Hz to 50 Hz. A special problem arises when it is necessary to create a PAL signal not from the original film, but from an NTSC signal telecined using the 3-2 pull-down system. Conventional standards conversion will result in artefacts arising from the artificial field sequence which is generated in the 3-2 pull-down system. A solution to this problem is provided in WO 91/06182 which describes a technique for recognizing and removing the duplicate fields before converting to the PAL line standard.
It is generally accepted that program material, originated in HDTV and then down converted to current broadcast standards, has a picture quality that far surpasses the quality available from xe2x80x9cnormalxe2x80x9d sources. Accordingly, an advantage is seen in the ability to create both NTSC and PAL masters from an HDTV telecined master, even where, say, a direct NTSC telecine of the film material in question, already exists. The NTSC and PAL masters produced in this way will be expected to have extremely high picture quality. If it can be arranged that the HDTV, NTSC and PAL masters have an inherently close match, there will be a considerable time and money saving in that the transfer will require to be approved only once by the director or other individual responsible for visual quality.
It is an object of a further aspect of the present invention to provide improved apparatus to this end.
Accordingly, the present invention consists, in a further aspect, in apparatus for the simultaneous creation of NTSC and PAL masters from an HDTV telecined master, comprising means for receiving a 3-2 pull-down phase signal indicative of the phase of the field processing in the telecine operation from 24 frames per second cinematographic film to 1125 lines and 60 fields per second (1125/60) high definition television (HDTV); down converter means receiving 1125/60 HDTV input and serving to line convert from 1125 to 525 lines, to field convert from 60 to 59.94 fields per second in dependence upon said phase signal and to create a modified phase signal including identification of dropped fields; and cross converter means receiving 525/59.94 input and to line convert from 525 to 625 lines and to field convert from 59.94 fields per second to 47.952 fields per second in dependence upon said modified phase signal.
In still further aspect, the present invention consists in apparatus for the simultaneous creation of NTSC and PAL masters from an HDTV telecined master, comprising first means, including a telecine, for producing from 24 frames per second cinematographic film, a high definition television (HDTV) signal at 1125 lines and 59.94 fields per second (1125/59.94) said first means further providing a modified 3-2 pull-down phase signal indicative of the phase of the field processing in the telecine operation and timed at 59.94 fields per second; down converter means receiving 1125/59.94 HDTV input and serving to line convert from 1125 to 525 lines; and cross converter means receiving 525/59.94 input and to line convert from 525 to 625 lines and to field convert from 59.94 fields per second to 47.952 fields per second in dependence upon said modified phase signal.